Structural Systems for Multifamily Construction

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Understanding available systems and the implications to your project’s size and cost.

Kinsley Construction, Inc. is a leader in multifamily construction, delivering over 3,000 units through our numerous projects in the Mid-Atlantic region. Each of these projects is unique in their design and often include structured parking, retail and assembly areas.

A challenge we see design professionals and clients encountering on multifamily projects is determining the right structural system for the project. The International Building Code (IBC) establishes the parameters of this decision, requiring that project design considerations include the distance from property lines and adjacent structures as well as the allowable building height and number of levels. Considering these factors, the most cost effective structural system or “bones” for the building can be determined early in the design and preconstruction phase of a project along with the advantages and disadvantages of each system.

EFFECT OF BUILDING HEIGHT & STORIES

In accordance with IBC 2015, a building is classified in one of five construction types based on the materials used for construction. For these construction types, IBC prescribes the required fire-resistance rating for structural elements such as primary structural frame, bearing walls, nonbearing walls, floor construction and roof construction. A Type I building classification has the most stringent fire resistant requirements for structural elements, and Type V construction has the least stringent. These construction types are sub-classified as Type A or Type B.

Structural elements for a Type A sub-classification have more stringent fire resistant rating requirements than a Type B sub-classification. IBC defines the allowable building height and number of stories for a new structure based on the type of construction, the use of the building (in multifamily construction the Use Group is generally R-2) and the type of fire suppression system required.

STRUCTURAL SOLUTIONS FOR CONSTRUCTION TYPES

IBC permits structural elements of any material, noncombustible or combustible, in Type V construction. Wood structural elements are a common solution for multifamily projects because they are readily available and are the most cost competitive structural solution. Walls and roof trusses can be prefabricated off site, expediting the on-site construction schedule. Wood structures allow for flexible designs and easy field modifications, are lightweight, and MEP costs are minimized compared to other structural systems.

In Type V construction, wood structural elements:

are limited to four stories and a 70’-0” building height

require supplementary systems to enhance acoustical performance

have limited span lengths

are subject to mold if not tested and treated in the field. It is best practice to warranty test and treat all wood-framed projects for mold.

Type III Construction

IBC requires the exterior walls be of noncombustible materials; the rest of the structural elements can be any material permitted by the IBC. Fire-retardant-treated wood framing is permitted in the exterior walls if the exterior walls have a two-hour fire rating or less.

As in Type V construction, structural elements made of wood are typical for Type III multifamily construction. However, for an R-2 occupancy the structure may be five stories and 85’-0” in height.

Type II Construction

IBC requires all structural building elements be of noncombustible materials. Type II construction is limited to five stories and 85’-0” building height for an R-2 occupancy. Multifamily projects with Type II construction have incorporated light gauge metal stud bearing walls.

Precast hollow core plank is readily available locally, does not require fireproofing, does not require shoring and the underside of the plank can be painted for a finished ceiling. Using precast hollow core plank allows for increased finish ceiling heights within the units as the depth of the plank is typically eight inches deep plus a cast-in-place topping slab. Pre-planning for penetrations through the plank is critical as it is difficult to make modifications in the field.

Metal deck with concrete composite slabs has positive qualities similar to those of precast plank. A precast plank system can be installed quicker than a metal deck/concrete slab system.

Both floor systems have better acoustical properties than wood. The underside of the metal deck is not typically left exposed. Light gauge metal stud walls allow for a better drywall finish as compared to wood and are much less susceptible to mold.

Type IB Construction

IBC requires all structural building elements are of noncombustible materials. Type IB construction is limited to twelve stories and 180’-0” building height for an R-2 occupancy. Light gauge metal stud bearing walls may be incorporated into the structural design, up to ten stories.

Precast concrete plank is cost effective and a viable floor solution up to seven stories.

An engineered joist/slab system consisting of a proprietary joist system with formed cast-in-place concrete slabs is another option. This system is versatile, simple, can achieve longer spans, and has positive fire and acoustical properties. This system has loading limitations, requires a finish ceiling below and shoring/reshoring.

There are proprietary light gauge metal stud bearing walls and joist/metal deck/concrete systems that can approach twelve stories but must be analyzed on a project by project basis.

RELATED GROUPS OR OCCUPANCIES

Many multifamily projects include structured parking or retail spaces below the building’s residential levels. The IBC requires a horizontal fire separation between differing uses of the building. Typically this is achieved by installing a podium slab or transfer slab that provides the required fire separation rating prescribed by the IBC and transfers the loads of the building above this slab down to the foundations. Generally, the most cost effective solution for the podium/transfer slab is a structural cast-in-place concrete system in lieu of a structural steel and concrete based system.

Of course, if all of a project’s parking requirements are addressed via on-site surface parking spaces and retail is not part of the project, slab-on-grade is the most cost effective solution.

Kinsley’s preconstruction services include participating in a design-assist role with the project team. We take a proactive approach to analyzing the available structural systems for a project and collaborating with the project team to decide which structural system is the best economical choice after considering all of the advantages of each system.

It is because of our unmatched experience with multiple structural systems for multifamily construction and our accurate comprehension of the International Building Code that we are considered the Master Builder for multifamily construction in the Mid-Atlantic region, delivering in every respect.